Jellyfish are found in every part of the sea, from the warm water at the surface to the cold, dark depths of the ocean. Jellyfish eat tiny sea creatures that drift into their tentacles. Jellyfish come in almost every colour of the rainbow and range in size from smaller than your fingernail to having tentacles that are many metres long. Some jellyfish are poisonous and their sting can kill a person. It is best to avoid touching a jellyfish if you don't know what sort it is.